1. Field of the Invention
The present invention relates to an image-forming optical system, and particularly to an image-forming optical system including a refractive optical unit and a reflective optical unit.
2. Description of the Related Art
Image-forming optical systems forming an image of an object surface on a predetermined surface are used in optical apparatuses such as video cameras, still cameras, copying machines, projectors and exposure apparatuses.
FIG. 8 shows an example of a reflective optical system as disclosed in Japanese Patent Application Laid Open No. 2000-231060. In this example, reflective surfaces 102 to 105 having a decentration are used, and an optical system with a compact structure is attained by letting the optical path cross within the optical system.
Ordinarily, it is possible to suppress chromatic aberration by using the reflective surfaces, so that by using a reflective optical system which is a non-coaxial optical system for projectors or image-scanning optical systems of copying machines, in which chromatic aberrations are problematic, it is possible to a realize an optical system with better performance than in the case of using a coaxial optical system.
Such a non-coaxial optical system is also referred to as an off-axial optical system. The off-axial optical system is defined as, when the path which a light ray passing through the center of an image and the center of a pupil is taken as the reference axis, an optical system including curved surfaces (off-axial curved surfaces) whose surface normals on the intersection between the surface and the reference axis do not coincide with the reference axis. In this case, the reference axis has a bent shape.
The constituent surfaces of such an off-axial optical system are ordinarily non-coaxial, and it is easy to construct the optical system using reflective surfaces, since no eclipse occurs at the reflective surfaces. Moreover, by forming an intermediate image within the optical system, it is possible to configure a compact optical system while ensuring a broad field angle.
Japanese Patent Application Laid Open No. H08 (1996)-292372, Japanese Patent Application Laid Open No. H09 (1997)-222561, and Japanese Patent Application Laid Open No. H09 (1997)-258105 disclose zoom optical systems using an off-axial optical system, and Japanese Patent Application Laid Open No. H09 (1997)-5650 discloses a method for designing a zoom optical system.
Moreover, optical systems combining refractive lenses and refractive surfaces have been proposed (see for example Japanese Patent Application Laid Open No. H09 (1997)-258106), but the object of most of these is to correct chromatic aberration.
In the optical system disclosed in FIG. 8, all of the reflective surfaces 102 to 105 are provided with a positive optical power in order to let the optical path intersect within the optical system and to make it compact. Furthermore, an incident surface 101 and an emergent surface 106 have a soft curvature and are substantially planar in order to suppress chromatic aberration. In order to suppress chromatic aberration, it is preferable that the incident and the emergent surfaces 101 and 106 have a negative optical power. In the case of refractive lenses, the Petzval's terms have the same sign as the optical power, whereas in the case of reflective surfaces, the Petzval's terms have the opposite sign of the optical power.
In the optical system of FIG. 8, if the influence of the incident and the emergent surfaces is disregarded, the Petzval's terms are all negative, so that a curvature of field occurs at the image surface, and a high optical performance cannot be attained. Moreover, if the optical system has reflective surfaces with a negative optical power, then the light flux diverges at these surfaces, so that the optical system becomes large. Due to this reason, it is desirable that the number of surfaces having negative optical power is reduced, when the goal is to make the optical system more compact.
However, in order to make the Petzval's sum zero, reflective surfaces having positive and negative optical power are necessary. As noted above, for refractive lenses, the Petzval's terms have the same sign as the optical power, where as for reflective surfaces they have the opposite sign of the optical power, so that when combining reflective surfaces and refractive lenses, it is possible to reduce the number of surfaces having negative optical power, making it possible to correct the Petzval's sum.